In color-gradient printing an applicator applies printing ink to an ink-transfer roll and has an ink supply with an outlet slot extending the full width of the ink supply and from which ink is emitted and applied to an ink-transfer roll. The ink is applied to the ink-transfer roll so as to produce color-gradient printing using at least two different printing inks.
This type of ink supply typically has a narrow side and a wide side, the wide side extending along and parallel to the rotational axis of the ink-transfer roll. The outlet is delimited by two narrow end walls of the ink supply and a long downstream doctor blade and a long upstream doctor blade that both contact the rotating ink-transfer roll. As a result, the outlet is closed by the outer surface of the ink-transfer roll where, due to the rotation of the ink-transfer roll, the printing ink in the ink supply forms a counter-rolling ink mass or bank from which printing ink passes to depressions in the ink-transfer roll and is transferred onto another roll, e.g. another ink-transfer roll or a roller carrying a printing plate.
An approach for generating color-gradient printing using these ink supplies has been known for some time, e.g. as a security feature. Here, for example, at least two different printing inks in a printing unit are applied side-by-side to the outer surface of an ink-transfer roll such that the printing inks touch or overlap, at least at their borders, and the printing inks mix together on the ink-transfer roll by means of suitable additional distributing rollers acting on the outer surface of the ink-transfer roll.
As a result, continuous color gradients are created at the border overlap regions of the adjacent printing inks, which gradients can be transferred, for example, by a printing plate onto a substrate. The motifs thus printed then have a corresponding color gradient with continuous color transitions since in this case a genuine mixing together of printing inks occurs.
Depending on the implementation of the ink supplies employed, it is also possible to use multiple different printing inks, thereby enabling the system to effect, for example, a rainbow-like color gradient. In this case, one ink supply of the known type has, for example, multiple side-by-side compartments holding different printing inks that are separated from each other by partitions. The regions here each have an outlet directed toward an ink-transfer roll, the respective printing ink being transferred to a specific portion of the ink-transfer roll.
It may be advantageous here to provide a common doctor blade for all the side-by-side ink supplies, thereby enabling a homogeneous application of ink over the entire outer surface of the ink-transfer roll to be achieved. In addition, it is possible for a mixing zone to be created, at least immediately at the edges of the doctor blades inside the ink supply at the borders of adjacent areas, thereby enabling a first continuous mixing together of different adjacent printing inks to be effected.
The further mixing together or distribution of the printing inks applied to the ink-transfer roll is subsequently effected by one or more distributing rollers that act on the surface of the ink-transfer roll and, for example, thus mixing together the printing inks at varying rotational speeds and/or using supplemental axial movements of the rollers.
However, a disadvantageous aspect here is that when using ink supplies of the known type a color gradient can be generated essentially only in one direction of the ink-transfer roll. Another disadvantage is that when using multiple printing inks in a common ink supply essentially only adjacent inks can be mixed together, with the result that a variable mixing of inks, and, in particular, a selective variable mixing of ink during operation is impossible.
Specifically, if, for example, three different process-printing inks—cyan, magenta and yellow—are disposed side-by-side in the referenced sequence in the referenced ink supply, the adjacent printing inks cyan and magenta, or magenta and yellow, can be mixed together on an ink roller—not however, the cyan and yellow printing inks. In addition, specially fitted ink supplies with partitions must be employed, with the result that existing printing units cannot be readily retrofitted.
Since the mixing together of inks increases continuously in this type of printing, and thus the mixing zone between the different printing inks on the printing roller is continually widened, while additionally already-mixed inks pass via the known back-transport of printing ink from the rolling ink mass into the ink supplies, the printing roller and ink supplies must be cleaned continuously, or at least at regular intervals, a process that can result in a not insignificant loss of ink in the printing machine. Color-gradient printing is therefore preferentially used in producing security papers in, for example, rotary offset printing presses or gravure printing presses.
An additional disadvantage of the known described type of color-gradient printing is that a series of additional components are required inside the printing unit—such as distributing rollers, cleaning devices, and their corresponding drive units and control system—due to the desired and requisite distribution of printing inks on the printing roller and to the continuously required cleaning of the printing roller, by which means a stable printing operation can be ensured. This increases the complexity of the printing unit and thus encumbers its operability, but also requires additional space for the requisite ancillary components, which factors, for example, impede or make it impossible to retrofit an existing printing machine with a color-gradient printing unit.
It is impossible to reproduce these types of continuous color transitions with the known printing techniques such as, for example, offset printing, flexographic printing, or even gravure printing, since these other techniques use a subtractive superimposition of multiple color separations of a printed image printed in succession on a substrate to reproduce a certain color impression, which aspect becomes clearly visible at least when an image thus printed is enlarged. What results from these techniques is thus an overlay—not mixing—of multiple colors.